1. Field of the Invention
The invention relates generally to the field of marine seismic survey apparatus. More particularly, the invention relates to “stretch sections” used in marine seismic streamer systems to reduce noise.
2. Background Art
Marine seismic survey apparatus known in the art include arrays of seismic sensors disposed in a structure adapted to be towed by a seismic vessel through a body of water, such as a lake or the ocean. Such seismic receiver structures are known as “streamers.”
Streamers are typically made in segments of about 75 meters length. A streamer may include 100 or more such segments coupled end to end to form the complete streamer. Each streamer segment generally include one or more steel or high strength fiber rope strength members that extend the length of the streamer segment. A commonly used fiber is one sold under the trademark KEVLAR, which is a registered trademark of E.I. du Pont de Nemours & Co., Wilmington, Del. The one or more strength members typically include buoyancy devices at spaced apart locations along the length of the strength members. The buoyancy devices may be made from foamed polyurethane or the like and help provide the streamer with an overall density similar to that of the water in which the streamer is to be towed. Electrical and/or optical sensors are disposed along the length of the strength member at spaced apart locations, typically in some of the buoyancy devices. Electrical and/or optical cables extend along the length of the streamer and are coupled to the sensors so as to transmit signals generated by the sensors in response to seismic energy to a recording device, which may be on the seismic vessel or at another location. Other conductors may be used to transmit electrical power. The streamer segment is typically covered with an acoustically transparent, flexible jacket, made from a material such as polyurethane and the like. The interior of the jacket is typically filled with an acoustically transparent, electrically non-conductive material such as oil, or curable polyurethane gel. The streamer segment typically includes a combination mechanical and electrical/optical coupling at each of its axial ends so that the streamer segment can be coupled to another such streamer segment or, through a stretch section, to a “lead in” cable, explained further below, coupled to the seismic vessel. The coupling transfers axial force from segment to segment and ultimately to the seismic vessel through the lead in cable.
In a typical seismic survey acquisition system, one or more streamers made as described above are towed behind the seismic vessel in the water. In acquisition systems having more than one streamer, the streamers are typically laterally separated from each other by coupling their forward ends at spaced apart positions to a “spreader cable” that extends transversely to the direction of motion of the seismic vessel. A device called a “diverter” is positioned at each end of the spreader cable. The diverters include vanes that cooperate with the movement of water during towing to exert a laterally outward force with respect to the direction of movement of the seismic vessel, and thus, maintain the spreader cable in tension.
The “lead in” cable includes a plurality of electrical and/or optical conductors that are essentially completely surrounded by one or more layers of helically wound steel wires. The steel wires are referred to as “armor” and protect the conductors from damage, and transmit axial load from the each of the streamers to the seismic vessel.
A particular issue that concerns marine seismic survey acquisition systems known in the art is a type of noise created by movement of the water past the lead in cable and the spreader cable. The noise is known as “strumming” and such noise can be of a nature so as to materially adversely affect the quality of the seismic signals detected by the sensors in the streamers. Other types of noise that affect streamers include mechanically generated noise in the diverters and fluctuation in the towing speed caused by variations in water conditions. One device known in the art for reducing transmission of such noise between the lead in cable and the streamer is known as a “stretch section.” A stretch section is typically formed similarly to a streamer segment as described above, with the principal differences being that the strength member in the stretch section is typically formed from a more elastic material than that used for streamer segments, and that there are no seismic sensors in the stretch section. An example of a material used for stretch section strength members is nylon rope. Typically, the stretch section is coupled between the lead in cable and the forward axial end of the streamer. Another type of device for reducing such noise transmission is called a vibration isolation module. A vibration isolation module is typically configured as a solid, elastomer cylinder of a selected length coupled between the forward end of the streamer and the lead-in cable. Vibration isolation modules known in the art are subject to tearing of the elastomer, and subsequent failure of the module.
Typical stretch sections are on the order of 50 meters in length. One or more such stretch sections may be coupled between the lead in cable and the forward end of the streamer. When more than one stretch section is used, the sections are coupled end to end to produce a longer stretch section. The total length of stretch section used in any particular seismic survey system will depend on the length of the streamer coupled to the stretch section. While they are effective in reducing strumming noise in the sensor signals, stretch sections do cause the nearmost seismic sensor to be spaced a considerable distance from the seismic vessel because of their length. Such distance increases the minimum offset, such being the distance between a seismic sensor and a seismic energy source towed in the water. Also, the construction of the typical stretch section results in such stretch sections requiring similar type and frequency of maintenance as a streamer segment. Such maintenance can be difficult and expensive. It is desirable to have a stretch section which can minimize the additional length to the streamer, avoids the failure modes of vibration isolation modules, and can be easily repaired in the field.